Monitoring fluid velocity using Active - Distributed Temperature Sensing (A-DTS)

Abstract:

Fluid flow logs measured in boreholes provide information on the location and quantity of water from productive regions or fractures intersecting a well. For many applications, the flow may be time varying, in which case many logs, possibly in quick succession, would need to be taken. Recent field studies by our team have shown the potential of Active-Distributed Temperature Sensing (A-DTS) methods to produce high-frequency fluid flow logs. In the A-DTS method, resistive heating is applied across the conducting materials in the cable, uniformly warming the cable. When installed in a flowing fluid, the DTS measured temperature in a heated cable is a function of the flow velocity, since a faster flowing fluid will more effectively cool the cable.

A flume trial was conducted to test the sensitivity of the A-DTS method for fluid velocity measurement in a controlled environment. The fiber-optic cable was fixed inside a test chamber through which the flow was channelled, with the velocity measured by Ultrasonic Doppler Velocimeters. The laboratory set-up allowed for the key variables to be examined. These included the heating rate (power per unit length of heated cable), and incidence angle of the flow to the cable.

The coupled fluid flow and heat transfer was numerically modelled. In addition to establishing the temperature-velocity relationship for a range of heating rates and cable configurations, the numerical model allowed the design of the cable itself, including material thermal properties and thicknesses, to be tested. The results should allow the sensitivity of subsequent A-DTS systems constructed for fluid flow monitoring to be optimised through appropriate cable materials and positioning in the borehole.